Pneumonia

The list of organisms which can cause pneumonia is very large, and includes nearly every class of infecting organism: viruses, bacteria, bacteria-like organisms, fungi, and parasites (including certain worms). Different organisms are more frequently encountered by different age groups. Further, other characteristics of the host may place an individual at greater risk for infection by particular types of organisms.

Viruses, especially respiratory syncytial virus, parainfluenza and influenza viruses, and adenovirus, cause the majority of pneumonias in young children. Pneumonia in older children and young adults is often caused by the bacteria-like Mycoplasma pneumoniae. Adults are more frequently infected with bacteria (such as Streptococcus pneumoniae, Hemophilus inflenzae, and Staphylococcus aureus).

The parasite Pneumocystis carinii is an extremely important cause of pneumonia in patients with immune problems, such as patients being treated for cancer with chemotherapy, or patients with AIDS. People who have reason to come in contact with bird droppings, such as poultry workers, are at risk for pneumonia caused by the parasite Chlamydia psittaci. A very large, serious outbreak of pneumonia occurred in 1976, when many people attending an American Legion convention were infected by a previously unknown organism (subsequently named Legionella pneumophila) which was traced to air conditioning units in the convention hotel.